In recent years, with the aim of reducing the amount of carbon dioxide emissions and improving fuel consumption, there has become widespread vehicle mounted with an electric motor and an internal combustion engine, that is, a hybrid vehicle and/or an electric car which is mounted with only an electric motor and drives by the driving force of the electric motor. In these vehicles mounted with the electric motors, there are mounted with a charging device which outputs direct current (DC) power, a power converter (inverter) which converts the DC power from the charging device into alternating current (AC) power and supplies the power to the electric motor, and the like, in addition to the electric motor.
The power converter which supplies the power to the electric motor converts the DC power into the AC power three-phase modulation drive pulse width modulation (PWM) control that uses switching elements such as insulated gate bipolar transistors (IGBTs).
Here, three-phase modulation drive and two-phase modulation drive of the power converter for use in such an electric motor control device will be described.
FIG. 13 is a chart showing the relation between a phase voltage command and a switching signal during three-phase modulation drive and during two-phase modulation drive.
In the case of generating a line voltage (A) and a motor current (B) shown in FIG. 13, a phase voltage is shown like (C) and an upper switching signal is shown like (D) in a driving method by the three-phase modulation; and a phase voltage is shown like (E) and an upper switching signal is shown like (F) in a driving method by the two-phase modulation. Incidentally, a triangular wave cycle (carrier frequency) is set to the same cycle.
As can be seen from FIG. 13, it shows that a switching operation is normally performed during the three-phase modulation drive, whereas there exist a switching signal that becomes normally ON (or normally OFF) and switching operation is small during the two-phase modulation drive.
There exist a loss at the time of start-up, an ON loss, and a loss at the time of start-up/stop in the switching operation of the switching elements. Accordingly, the switching operation is small means that a loss generated by power conversion from DC power to AC power is small.
However, the power conversion from the DC power to the AC power is the switching operation at high frequency and high power; and accordingly, the switching element generates heat by a loss (switching loss) generated by the switching operation and the temperature of the switching element rises. Then, when the switching element reaches its upper limit temperature, supply power to the electric motor is reduced is order to avoid a breakdown of the switching element and, more specifically, the number of switching is reduced and it becomes difficult to supply desired power, which is for generating driving force corresponding to a driver's operation, to the electric motor.
As a section of preventing a deterioration of supply power caused by such a temperature rise of the switching element, that is, an output deterioration of the electric motor, there is a technique disclosed in Patent Document 1.